1. Field of the Invention
The present invention relates to a zoom lens system. More particularly, the present invention relates to a zoom lens system having a large zoom ratio and applicable to a compact camera such as a video camera, an electronic still camera, etc.
2. Prior Arts
Recently, due to the packaged electronic parts and the improvement of the integration rate, more and more camera bodies such as those of video cameras have been made compact in both weight and volume. In addition, the price and cost of the camera have been rapidly decreasing. However, the relative rate of the weight, volume, and cost of a lens system to those of a camera system is increasing year by year although the absolute rate is decreasing little by little. Under such circumstances, a demand for a low-priced compact camera is increasing.
On the other hand, a lens system has been requested to have higher functions such as a large aperture ratio for compensating for a short of illumination due to a small-sized photographing device and an improved aberration performance for a high picture element and a high resolution.
Currently, a zoom lens system with a zoom ratio of approximately 6.times. is mainly used for a video camera. As a zoom lens system with such high zoom ratio and having a large aperture ratio with an F-number of approximately F1.6, various zoom lens systems with four or five lens units have been conventionally proposed. However, most of the zoom lens systems have 13 to 15 lens elements, which makes it impossible for them to meet the above-described demand for a low-priced compact camera.
To decrease the number of lens elements of a zoom lens system, an aspheric surface has been widely used recently.
For example, a zoom lens system disclosed in Japanese laid-open Patent Application S57-27219, which does not have a zoom ratio of 6.times., consists of a positive, a negative and a positive lens units from the object side. In the zoom lens system consisting of twelve lens elements and having a zoom ratio of 3.times., the first lens unit for compensating for a position of an image point (compensator) and the second lens unit for zooming (variator) move along an optical axis and each unit has an aspheric surface.
However, the configuration and arrangement of lens elements of the zoom lens system are not effective, and the number of the lens element is still too many. Also, it is impossible to increase the zoom ratio of the zoom lens system to approximately 6.times., because of the following defect of the zoom lens system as well as the above-described inadequate configuration and arrangement of the lens elements.
That is, since the third lens unit does not move at a zooming operation, naturally the first lens unit is forced to move as a compensator lens unit. When a zoom ratio of approximately 6.times. is attained under above condition, the first lens unit largely moves toward the object side at a shortest and an intermediate focal length conditions. Because of this, the diameter of the front lens becomes larger than that of the front lens of a lens system having four or five lens units, resulting in that the entire lens system becomes considerably heavy.
On the contrary, Japanese laid-open Patent Application No. S61-110112 and Japanese laid-open Patent Application No. S60-107013 disclose four-unit zoom lens systems in which lens elements and aspheric surfaces are effectively arranged to largely reduce the number of lens elements constituting the zoom lens system.
In Japanese laid-open Patent Application No. S61-110112, a four-unit zoom lens system having a positive, a negative, a negative and a positive lens units, from the object side, which are simply arranged attains a zoom ratio of 6.times. with only eight lens elements by effectively using four aspheric surfaces.
Although having an excellent lens arrangement, it is difficult for the zoom lens system to meet the above-described demand because of its poor aberration performance.
In Japanese laid-open Patent Application No. S60-107013, a four-unit zoom lens system consisting of a positive, a negative, a positive and a positive lens units, form the object side, is shown. And the zoom lens system has eight lens elements in total. Although its performance and size cannot be judged since no numerical data are disclosed, it is obvious that the zoom lens system does not satisfy the current demand for a high zoom ratio and a large aperture ratio, because the lens system has a specification of a zoom ratio of 4.times. with an F-number of F2.0.
Japanese laid-open Patent Applications No. S63-304218, No. S64-44907 and No. H1-223408 propose three-unit zoom lens systems consisting of a positive, a negative and a positive lens units from the object side, where the first lens unit has one or two lens elements and the second lens unit has one lens element, and aspheric surfaces are employed to reduce the number of the lens elements. In the zoom lens systems, the second lens unit, which plays the most important role and moves along an optical axis at a zooming operation, consists of one single lens element having a negative refractive power. For reason of this, since the chromatic aberration is not corrected in the second lens unit in the zoom lens systems, the variation in chromatic aberration by a zooming is large, so that their performances cannot be guaranteed at a zooming with a high zoom ratio. These zoom lens systems realize zoom ratios of only 2.times. to 3.times. with F-numbers of only 2 to 4. It is impossible to prevent the above variation in chromatic aberration even by employing aspheric surfaces. Also, judging from the current required performance (including chromatic aberration), it is apparent that a zoom lens system having such lens arrangement can realize a zoom ratio of 3.times. at most; it is difficult to realize a zoom ratio of approximately 6.times..
Japanese laid-open Patent Applications No. S64-91110 and No. H1-185608 disclose novel zoom lens systems.
Japanese laid-open Patent Application No. S64-91110 discloses a three-unit zoom lens system having almost the same lens configuration and arrangement as the zoom lens system which the Applicant of the present invention proposed in Japanese laid-open Patent Application No. S64-88511. Although the zoom lens system has only three lens units, it works as a four-unit zoom lens system since the second lens unit is divided into two lens group: negative lens group consisting of two negative lens elements and a positive lens group consisting of a positive lens element. It realizes a zoom ratio of 3.times. with only 8 to 11 lenses which are usually applied to a three-unit zoom lens system. A zooming operation is performed by the independent movements of the negative (substantial second unit) and the positive (substantial third unit) lens units. However, in the four-unit zoom lens system, since the chromatic aberration is not completely corrected in the second and third units which independently move, the variation in chromatic aberration can not be fully prevented at a high zoom ratio because of the change of the relative position of the second and third units. In this zoom lens system, the variation in chromatic aberration is prevented by specifying a zooming while maintaining a zoom ratio of 3.times.. However, it is considerably difficult to realize a zoom ratio of 6.times. in the zoom lens system.
Japanese laid-open Patent Application No. H1-185608 discloses a zoom lens system where a zoom ratio of 6.times. is realized while having a fewer number of lenses than the zoom lens system proposed in Japanese laid-open Patent Application No. S64-91110 by employing many aspherical surfaces. In the zoom lens system, the second lens unit consists of a negative single lens element and the third lens unit consists of a positive single lens element, and the fourth lens unit is also simplified. Even in this zoom lens system, it is difficult to realize a currently required performance since the chromatic aberration cannot be completely corrected due to a large variation in chromatic aberration even though a zooming is fully specified. In addition, to correct the chromatic aberration, the movement amount of the second and third movable lens units should be largely increased, which makes the total length of the lens system longer. Especially, the diameter of the front lens is larger than that of general zoom lens systems of the same specification. From this respect, the zoom lens system cannot achieve a compactness. The zoom lens system disclosed in Japanese laid-open Patent Application No. H1-185608 has achieved an object of reducing the number of lenses. However, it does not meet the current demand in compactness and chromatic aberration performance.
Japanese laid-open Patent Application No. H2-39011 discloses a four-unit zoom lens system having a positive, a negative, a positive and a positive lens units from the object side and where the variation in chromatic aberration is prevented. In this zoom lens system, three aspheric surfaces are employed, and a zoom ratio of 6.times. with a F-number of F1.4 is realized with eight lens elements. The zoom lens system possibly meets the current demand in cost, performance and size; however, it does not meet the demand in weight since the diameter of the front lens is not small enough. Also, in the zoom lens system, the coma in a sagittal direction which is not fully expressed in an aberration diagram is very large, and therefore the off-axial performance greatly deteriorates.
On the other hand, to attain a compactness and to reduce the number of lens elements in a zoom lens system, the Applicant proposed three-unit zoom lens systems (positive, negative and positive) having only eight lens elements where no aspheric surfaces are employed in Japanese laid-open Patent Applications No. S64-74519 and No. S64-74520. Although these zoom lens systems are very compact and have an excellent performance, the required zoom ratio is not realized; the zoom ratio achieved by the zoom lens systems is only 2.times. to 3.times..
Japanese laid-open Patent Applications No. S54-30855, No. S54-80143 and No. H2-39116 disclose three-unit zoom lens systems (positive, negative and positive) for a single-lens reflex camera and a compact camera where each unit is moved to reduce the number of lens elements and to realize a high zoom ratio. The zoom ratio is 2.4.times. with ten lens elements in said S54-30855, 3.times. with eleven in said S54-80143, and 3.times. with twelve in said H2-39116. As described above, the required zoom ratio is not realized; especially, the second and third lens units are not fully simplified, and the demand for a low cost is not met.
By the way, users require a camera with a wide angle, a high zoom ratio and a large aperture ratio needless to say with a compactness and a low price.
A camera of a standard specification has a zoom lens system of 2.omega.=50.degree. (at the shortest focal length condition) with a minimum F-number of 1.4 to 1.8. For consumer lens system, zoom lens systems with an F-number of approximately 1.2 or those with zoom ratios of 8.times., 10.times., 12.times. and 16.times. by increasing focal length at the longest focal length condition where lens elements are allowed to increase in size are also provided. However, these lens systems are large in size and a low cost is not realized.
With respect to a requirement for a wide angle, a zoom lens system having an angle of view of 2.omega.=60.degree. which is very effective for an indoor photographing is strongly required since no such zoom lens system is provided for consumer.
The following are the reasons why a zoom lens system having a wide angle of view has not been proposed.
It is relatively easy to increase a zoom ratio of the above-mentioned zoom lens system of a standard specification (2.omega.=50.degree., a zoom ratio of 6.times., and a minimum F-number of 1.4 to 1.8) by increasing the focal length at the longest focal length condition.
FIG. 1 shows a zoom lens system proposed in Japanese laid-open Patent Application No. S63-305317, which is enlarged to an adequate size. In FIG. 1, the movements of each lens unit from the longest (T) to the shortest (W) focal length conditions are shown by the arrows. The prior art shown in FIG. 1 is an 8.times. zoom lens system of 2.omega.=50.degree. with an F-number of 1.7. It is not so much larger in size and number of lens elements than a 5.times. zoom lens system.
However, in the zoom lens system, it is very difficult to increase an angle of view at the shortest focal length condition because of the following reasons (a) and (b).
(a) Because of the wide angle of view, the off-axial light is eclipsed by the front lens to cause a short of illumination unless a comparatively large front lens is provided. To prevent the short of illumination, a very large front lens is required, so that the weight of the zoom lens system is largely increased.
(b) Since the off-axial light enters at a considerably large angle, the distortion and the field curvature greatly deteriorates. Therefore, a greater number of lens elements are required to correct the aberration, which increases the cost.
As a specific example, a zoom lens system of a little different specification is shown as there are no zoom lens system having a wide angle that can be shown as an example. FIG. 2 shows a zoom lens system proposed in Japanese laid-open Patent Application No. S54-17042. In FIG. 2, the arrows show the movements of each lens unit from the shortest (W) to the longest (T) focal length conditions. The zoom lens system has an angle of view of 2.omega.=56.degree., an F-number of 1.4 and a zoom ratio of 9.5.times.. With only a little wider angle of 2.omega.=56.degree., the size of the front lens apparently increases. What increases the size of the front lens is not so much an increased zoom ratio as a wider angle of view.
FIG. 3 shows a zoom lens system with a wider angle of view proposed in Japanese laid-open Patent Application No. S62-153913. The zoom lens system has an angle of view of approximately 2.omega.=83.degree. and a zoom ratio of 8.times.. This is a specification for professional use. Although such wide angle is not required for consumer use, the zoom lens system in FIG. 3 shows how difficult to realize a wide angle. In FIG. 3, the arrows show the movements of each lens unit from the shortest (W) to the longest (T) focal length conditions.
As described above, it is very difficult to achieve a wide angle of more than 2.omega.=50.degree.. To realize a wide angle, it is inevitable that the size and the cost are largely increased. In video cameras for consumer use for which a compact size and a low price are required, such wide-angle zoom lens system has never been employed.
With respect to video cameras, no such compact and low-cost wide-angle zoom lens systems have been found in the patent gazettes. On the other hand, with respect to single lens reflex cameras, some wide-angle zoom lens systems of approximately 2.omega.=60.degree. have been proposed, such as the zoom lens system proposed in Japanese laid-open Patent Application No. S62-270910 shown in FIG. 4. In FIG. 4, the arrows show the movements of each lens unit from the shortest (W) and the longest (T) focal length conditions.
However, even in such prior arts,a zoom cam is in such prior arts, required since the number of lens elements is quite large and four or more lens units non-linearly move along an optical axis at a zooming operation, which makes it impossible to reduce the cost and the size.
As described above, no compact and low-cost wide-angle zoom lens systems with a high zoom ratio have been proposed which meets the demand of users for a wider angle.